The present invention relates to an improved linear motor. More particularly, the present invention relates to a linear motor which utilizes a variable resistance voice coil to generate a linear motor while still achieving high efficiency.
Linear motors are extensively used today for transducers for audio equipment. In addition to use in audio equipment, linear motors have applications in machinery, transportation, weapons design, and mass accelerators.
As illustrated at FIG. 1, a standard Alternating Current (AC) motor system typically includes a steel return circuit, a t-yoke and a gap plate, and a magnetic source. The most common motors use ceramic magnets, but any magnetic source may be used. Steel is ideal for the return circuit because it has high magnetic permeability. The magnets generate a Magnetic Field (B). The flux path of the magnetic field generally contours along the steel path and across an air gap which exists between the gap plate and the t-yoke. The air gap is generally extremely small such to minimize loss through the less permeable air.
A voice coil, or coil, (a current carrying conductor) resides inside the non-uniform magnetic air gap. As current is driven through the voice coil, the coil experiences a force perpendicular to the current direction.
However, the magnetic field generated in the linear motor is not uniform. The non-uniform flux density of the xz-plane (orthogonal to the motion of the driver) is irrelevant for linear conductor studies because the flux lines are concentric about a symmetrical cylinder. Moreover, differences in Magnetic Field (B) on the Length (L) of the voice coil wire between the outside edge of a voice coil and the inside edge symmetrical balance out when summed across the entire coil.
However, a non-uniform field in the y-axis, which causes non-linear force as the coil moves through the fixed non-linear magnetic field, has a significant effect on voice coil fidelity and distortion. Thus, when the bottom or top of the coil nearly reaches the bottom or top of the gap, respectively, at ultra high displacement, the relative force generated on the active coil is drastically reduced.
This non-linear force problem arises when the voltage applied to the voice coil is high, or the period is very long (low frequency input) which, in either case, causes displacement beyond linear tolerances.
Many driver types have been designed to make the force on the voice coil more linear as a function of displacement. Traditional designs have included overhung and underhung type designs. Overhung refers to a shorter gap plate and a longer coil that extends beyond the gap at all times. In an underhung design there is a taller gap and a shorter coil that always sit inside this gap (never extends above or below). The idea is to create a difference between the coil and gap such that when the driver moves up and down, coupling between magnetic flux and coil can be maintained at all displacements at all times. All things being equal, overhung coils tend to have higher force factors, but underhung coils tend to be more linear but much less efficient when designed to have the same voice coil height to gap height differential.
In addition to these traditional designs, modern dual gap motors and variable coil motors have been designed to linearize force as displacement of the voice coil occurs. Examples of these motor designs include XBL2 TM designed by Senior Transducer Engineer Dan Wiggins, the LMS designed by Senior Transducer Engineer Thilo Stompler, and the Split Coil, an open source design. See FIGS. 13, 14 and 15 for examples of double gap, variable coil and split coil designs respectively.
While these current modern methods go far to generate relative linear force regardless of displacement, they do so at the expense of efficiency. For many audio applications low energy usage is extremely important. Thus, efficiency of the transducer becomes crucial. Likewise, reduced efficiency of a linear motor results in decreased sensitivity and lower overall output.
Hence there is a need for an improved linear motor system and method. Such a system may provide linear force on the voice coil regardless of displacement while maintaining a high degree of efficiency. Such a system may enable a linear motor with reduced distortion where reduced distortion motors were previously impractical due to low sensitivity.